In the metal forming arts, there are known a variety of stretch-forming techniques in which a workpiece, usually an elongated extrusion or a sheet-form member, is formed to the profile of a forming die surface in conjunction with the application to the workpiece of tension exceeding its yield point. The tension is applied along the line coinciding with the bend axis of the workpiece that is deformed during the forming process.
The known stretch-forming processes typically have been applied in the forming of aluminum alloy components such as elongated extrusion sections or thin section panels like those often used for aircraft fuselage skin.
One known stretch-forming process, for example, is often referred to as "stretch wrapping" or "stretch-wrap forming" and involves the application of mechanical tension to a workpiece to thereby stretch it during formation. Subsequently, while the tension is maintained, the workpiece is wrapped about a form die. When a hollow extrusion is bent over a form die without being under tension, the resulting compressive forces on the inner surface of the extrusion cause this inner surface to buckle or deform. The basic goal of stretch-wrap forming is to stretch the extrusion along its bend axis to a point where the inner surface will not deform when subsequently bent over a form die. Stretch-wrap forming may be well suited to forming a workpiece to long sweeping curvatures of liberal radii.
Other stretch-forming techniques include moving die arrangements, in which the gripping heads are stationary and the forming die is moved perpendicular into the workpiece. Another technique is radial draw forming, in which one grouping head and the die are mounted on a table that rotates to slowly draw the part under tension over the rotating die.
An advantage of the various stretch-forming processes may generally include a decrease or elimination of the buckling of the inner surface of the workpiece. Furthermore, the desired results may be achieved with only minimal reduction in workpiece section thickness, typically not exceeding a five percent reduction.
However, the above stretch-forming processes generally require an unacceptable amount of human interaction and may not be suitable for a mass production environment. Also, imparting additional bends or even twists in the extrusion typically requires additional, time-consuming process steps whereby the extrusion may be dismounted from one bending machine, remounted in another, and then bent again. These extra process steps slow down the overall extrusion forming process and can not easily be accomplished by an automated assembly line.
In view of the foregoing, a need has been recognized in connection with the provision of a multiple bend or twist apparatus and method for use in a mass production environment. The apparatus can be used with limited human interaction and is well suited to an automated environment. Generally, the apparatus and method may form more complex extrusions in one process step and in a decreased amount of time compared to current methods. The profiles of the various stretches, bends, and twists may be precisely controlled and need not be constant throughout the forming process.